Electronic flashes provide supplemental light for photography to enhance images captured by a camera or other imaging devices. Traditional electronic flashes utilize a bulb filled with gas, such as argon, krypton, neon and xenon, or vapor, such as mercury vapor. When a high voltage is applied to the bulb, the gas or vapor is ionized, allowing electrons to flow through the gas or vapor. These electrons excite the atoms of the gas or vapor, which emit light. The wavelength characteristics of the emitted light depends on the gas or vapor in the bulb. In the case of mercury vapor, the emitted light is ultraviolet light, which is usually converted to visible light using fluorescent material since ultraviolet light is typically not desired.
Recently, light emitting diodes (“LEDs”) have been improved to a point with respect to operating efficiency where LEDs are now replacing conventional light sources, even bulbs in electronic flashes. Existing LEDs can emit light in the ultraviolet (“UV”), visible or infrared (“IR”) wavelength range. These LEDs generally have narrow emission spectrum (approximately +/−10 nm). As an example, a blue InGaN LED may generate light with wavelength of 470 nm +/−10 nm. As another example, a green InGaN LED may generate light with wavelength of 510 nm +/−10 nm. As another example, a red AlInGaP LED may generate light with wavelength of 630 nm +/−10 nm. However, since electronic flashes typically need to produce white light for color rendering purposes, different color LEDs such as red, blue and green LEDs are used together in an electronic flash to produce a white flash of light.
LED electronic flashes are commonly used in compact digital cameras with complementary metal oxide semiconductor (CMOS) image sensors. In these CMOS cameras, the LEDs of the electronic flashes are driven in continuous mode during an integration (exposure) period, i.e., the LEDs are turned on for the entire integration period. However, due to their architecture, CMOS cameras read out information sequentially, pixel row by pixel row. Hence, only after the image information in one pixel row is read out, the information in the next pixel row is read out. As a result, the integration time for each pixel row is staggered in order to maintain the same integration time for all the pixels in the CMOS image sensor to capture an entire image.
FIG. 1 illustrates the staggered integration technique to sequentially read out information from pixels rows 10(1), 10(2), 10(3) . . . 10(N-2), 10(N-1) and 10(N) of a CMOS image sensor. In FIG. 1, the integration time 12 and the readout time 14 for each pixel row of the CMOS image sensor are shown. As shown in FIG. 1, the readout time 14 for each pixel row begins after the end of the readout time for a previous pixel row, except for the first pixel row 10(1). Consequently, the integration time 12 for each pixel row is staggered with respect to adjacent pixel rows so that the integration time is the same for all the pixel rows. Therefore, the total integration period to capture an entire image begins when the integration time 12 for the first pixel row 10(1) begins at t=t1 and ends when the integration time for the last pixel row 10(N) ends at t=tn.
Since LEDs of an electronic flash for a CMOS camera are driven in continuous mode, the LEDs must be turned on during the entire integration period to produce a flash of light 16 with a predefined intensity I. However, as shown in FIG. 1, the light 16 from the LEDs of the electronic flash is not used by most of the pixel rows of the CMOS sensor near the beginning and the end of the integration period. For example, from t=t1 to t=t2, the light 16 from the LEDs of the electronic flash is used only by the pixel row 10(1) of the CMOS image sensor. Thus, the light 16 from the LEDs of the electronic flash is not used efficiently for the CMOS image sensor.
In view of this concern, there is a need for an imaging device and method for producing a flash of light using LEDs that more efficiently uses the light generated by the LEDs.